feline hyperthyroidism Flashcards
feline Hyperthyroidism
- how common?
- age?
- usual cause?
Most common endocrine disease of middle aged-older cats
◦ Mean age at presentation is 12-13 y
◦ <5% are <8 years at diagnosis
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Disease in one/both lobes of thyroid gland
◦ Usually adenomatous hyperplasia
◦ Overproduction of thyroid hormone
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Pathogenesis uncertain
◦ Risk factors – genetic, environmental, nutritional, other?
hyperthyroid feline clinical presentation - common and less common clinical signs
COMMON:
* Weight loss
* Polyphagia
* Unkempt haircoat
* Polyuria/polydipsia
* Increased activity, “nervous”
* Intermittent vomiting/diarrhea
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LESS COMMON:
* Lethargy
* Decreased appetite
common and less common PE abnormalities for hyperthyroid feline:
COMMON:
* Palpable thyroid
* Decreased BCS
* Hyperactive, impaired tolerance to stress
* Tachycardia
* Hair coat changes (dry, dander, alopecia)
* Heart murmur
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LESS COMMON:
* Arrhythmia, heart murmur
* Hypertension
* Lethargy, weakness
* Small kidneys
“Apathetic” Hyperthyroidism - how common? what is this?
10% hyperthyroid cats “apathetic”
* Depressed
* Anorexia
* Weight loss
* May have concurrent illness (e.g., renal, heart disease)
hyperthyroid feline Laboratory Results
CBC
* Usually normal
* Possibly increase in PCV / RBC numbers and increase in MCV
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Biochem
* Mild to moderate elevation in liver enzymes (90% of cats)
* Signs of concurrent kidney disease possible
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Urinalysis
* Usually normal, unless other diseases present
Diagnosing Hyperthyroidism - tests
Total T4 extremely specific
* 10% have normal T4 (typically upper end of normal)
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Other diagnostics:
* Investigate for concurrent disease
* Investigate for consequences of hyperthyroidism
feline hyperthyroid concurrent conditions
◦ Heart disease
◦ Hypertension
◦ Chronic Kidney Disease
Thyrotoxic Cardiomyopathy - what is this? clinical signs?
- Hypertrophic cardiomyopathy secondary to excess thyroid hormone levels
- Dilated cardiomyopthy less common
- Congestive heart failure possible, but seems uncommon
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Clinical signs: - Tachycardia
- Less commonly: murmur, arrhythmia, signs of CHF
Systemic Hypertension relation to hyperthyroidism? clinical signs? possible sequelae?
can we reverse this?
- Due to excess thyroid hormone or concurrent kidney disease
- Usually clinically silent
- Could cause retinal hemorrhage, detachment (uncommon)
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Reversible, if only due to hyperthyroidism - Could require some temporary treatment of hypertension
- E.g.,BP > 180mmHg, or concerns of retinal changes)
Chronic Kidney Disease relationship to hyperthyroidism
- Hyperthyroidism and CKD both common conditions of older cats
- Can be present in the same cat
- Hyperthyroidism does not cause CKD
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Hyperthyroidism will increase blood flow to the kidneys
◦ Increased blood flow increases renal filtration ability
◦ Therefore could mask azotemia/signs of CKD - Once euthyroidism achieved
> True reflection of renal status
Therapeutic Options for hyperthyroidism
◦ Surgical
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◦ Medical
> Methimazole
> Radioactive Iodine
> Nutritional management – low iodine
Thyroidectomy - possible complications?
◦ Minimal complications
◦ Ectopic hyperplastic tissue – recurrence
Methimazole - what does it do?
Concentrated by thyroid gland
* Inhibit T3 & T4
* Potentially reversible
Methimazole Side Effects - when are they seen? what are they? how common?
Usually occur in first 4-8 weeks of treatment
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Gastrointestinal signs (~10% of treated cats)
* Vomiting, diarrhea
* Self-limiting (temporary dose decrease)
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Blood cell abnormalities – most commonly neutropenia (<10% treated cats)
* Usually resolves when medication discontinued
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Facial excoriation (<5% of treated cats)
* Scabbed lesions on head, pinnae & neck
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Hepatotoxicity (2% of treated cats)
* Increases in liver enzyme values, signs of illness
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Significant side effects will require change to another form of treatment
Radioactive Iodine (131I) - safety and effectiveness? admin strategy?
Safe, simple, effective
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Usually administered SQ (or IV)
◦ Cats isolated (7-10d)
◦ Variable protocol per facility
◦ Some regulatory changes have limited availability of this treatment at present
Nutritional Management for hypothyroidism
- considerations for diet
- speed of efficacy
Iodine restricted prescription diet
* Sole nutrition source
* Caution if other medications/supplements
* Compliance is critical
* Cannot go outdoors
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T4 slower to return to normal
* 8 weeks (versus 2-4 weeks with methimazole)
goal of methimazole therapy
◦ Goal is mid-range TT4 (15-40 nmol/L)
◦ Recheck in 2-4 weeks
CKD & Hyperthyroidism after thyroid issues are treated
- what is revealed
◦ Once thyroid levels are controlled, GFR returns to normal
◦ Unmasks the true kidney function
◦ ~20% of patients will show azotemia after hyperthyroidism is
controlled
◦ CKD was present all along, but masked by the elevated thyroid hormone level +/- cachexia
prognosis for hyperthyroid cats with azotemia before treatment vs those that develop azotemia after treatment
◦ Hyperthyroid cats with azotemia before treatment have poorer prognosis
> MST 178 days
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◦ Hyperthyroid cats that develop azotemia only after treatment
> Prognosis similar to those that don’t develop azotemia
> MST 905 days
Managing CKD & Hyperthyroidism - goals for TT4 levels
◦ Keep TT4 in middle of reference range
> Low-normal or low T4 will drop GFR, associated with poorer survival
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No advantage to keeping the TT4 at high levels
◦ Urea and creatinine will improve but true kidney status does not improve
◦ Prolonged hyperthyroidism likely damages kidney
Long-term Monitoring for hyperthyroid cat?
Hyperthyroid cats developing azotemia after treatment - are they candidates for permanent treatment options?
◦ Once TT4 is stable, decrease frequency of rechecks
◦ Monitor kidney status as per CKD guidelines, if applicable
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◦ Hyperthyroid cats developing azotemia after treatment still good candidates for permanent treatment options (e.g., I-131)
Prognosis: Hyperthyroidism
Survival times 2-5+ years
◦ Hyperthyroidism usually does not cause death
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Azotemia prior to treatment
◦ Decreases prognosis
Name a common drug that could cause low T4 levels in a dog without hypothyroidism
◦ Steroids
◦ Phenobarbital
◦ Sulfonamides
Name a common abnormality on the biochemical profile in dogs with hypothyroidism?
- Decreased total T4
- Increased cholesterol (~75% dogs)
- Increased triglycerides
- Increased liver enzymes (~30% dogs)
What is a common biochemical profile abnormality with hyperthyroidism, usually resolves with treatment of thyroid disease?
◦ Elevated ALT
◦ Hypertriglyceridemia
◦ Hypoglycemia
◦ Elevated ALT
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* Mild to moderate elevation in liver enzymes (90% of cats)
* Signs of concurrent kidney disease possible
Which of the following is a common side effect of methimazole therapy?
◦ Kidney damage
◦ Lameness
◦ Vomiting
Gastrointestinal signs (~10% of treated cats)
* Vomiting, diarrhea
* Self-limiting (temporary dose decrease)
